The present invention relates generally to the field of pollution control, and, more particularly, to removing nitrogen oxides (NOX) from effluent gases.
Nitrogen oxides (NOX) are criteria air pollutants which are emitted in large quantities from high temperature processing sources, such as fossil-fueled power plants, industrial boilers, waste incinerators, and manufacturing plants for the production of nitric acid, fertilizer, explosives, plastics, cement and metal products, for example. Two major constituents of NOX are nitric oxide (NO) and nitrogen dioxide (NO2), which are considered to be large contributors to smog, acid rain and other deleterious environmental effects when discharged to the atmosphere. The quantity of NOX which may be discharged by a source is (or is expected to be) generally limited by governmental regulations. Because of the environmental concerns posed by air pollution, much research time and money has been expended to develop methods for controlling NOX emissions.
The reduction of NOX emissions from motor vehicle engines has been relatively successful, using catalytic converters. Improvements resulting from further developments appear to have diminishing benefits and incur high installed costs.
Inasmuch as a large portion of flue gas NOX is generated at stationary sources, removal efforts in the United States and elsewhere are now being directed to significantly reduce such NOX emissions. Current government enforced emission limits, which are often difficult to meet, are expected to become increasingly more stringent.
Stationary fossil-fuel fired power plants comprise a major source of flue gas emissions which contain both sulfur dioxide (SO2) and nitrogen oxides (NOX). Currently, emissions of SO2 are much reduced at many coal-fired power plants by wet scrubbing of the flue gases with an alkaline water stream, but removal of NOX by scrubbing is largely unsuccessful. Nitric oxide (NO), a primary constituent of NOX, has a very low water solubility and is not amenable to water scrubbing. As shown below in a comparison of values of Henry""s constant in water at 25xc2x0 C., nitrogen dioxide NO2 has a much greater solubility than nitric oxide NO, and the nitrogen acids HNO2 and HNO3 are in turn much more soluble than NO2.
Values of Henry""s Constant H, atmosphere/mol fraction
Because scrubbing of NOX from fossil-fuel power plant flue gases is largely ineffective, current NOX control methods primarily comprise combustion modifications, e.g. burners which are controlled to either limit the quantity of NO which is formed or reduce NO and NO2 to elemental nitrogen gas N2. Typically, such methods reduce NOX emissions by only about one-half, generally far less than is required to meet governmental restrictions. Furthermore, the burners are relatively costly.
A variety of post-combustion NOX removal methods which have been used or proposed may be classified as Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR) or Non-Selective Catalytic Reduction (NSCR). The Selective Catalytic Reduction (SCR) method involves the use of a catalyst system which selectively converts NOX to elemental nitrogen N2, optionally using an added reductant such as ammonia, urea, etc. Examples of SCR methods are described in U.S. Pat. No. 5,520,895 of Sharma et al., U.S. Pat. No. 5,589,147 of Farnos et al. and U.S. Pat. No. 5,180,567 of Yoshimoto et al.
In a related process described in U.S. Pat. No. 5,489,421 of Van Velzen et al., NO in the flue gas is absorbed in a scrubbing liquor containing FeII-EDTA, desorbed and concentrated by vapor stripping, and catalytically converted to hydroxylamine.
SCR methods are used at only a few major power plant installations, because of very high capital costs and substantial operating expenses. The SNCR and NSCR methods have found little practical application because of low conversion efficiencies. It has been proposed in U.S. Pat. No. 5,120,508 of Jones to convert NO to nitrogen dioxide NO2 by injecting a peroxyl initiator and oxygen into a flue gas stream, and removing the NO2 from the treated flue gas with a particulate sorbent. The initiator is any of a great number of materials including (a) compounds containing only carbon and hydrogen, (b) compounds containing only carbon, hydrogen and oxygen, (c) compounds containing only hydrogen and oxygen, and (d) hydrogen H2. Test results cited in the reference show NO conversions of up to about 83 percent, using propane as the peroxyl initiator. There is no indication in this reference of required concentrations of other initiators, or their effectiveness. The use of hydrocarbon initiators is expensive and consumptive of natural resources.
There are various references to the use of hydrogen peroxide in removing NOX from various source gases. For example, U.S. Pat. Nos. 4,182,278 of Coakwell and 5,647,304 of Nyberg et al. describe methods and apparatus for improving gasoline mileage and reducing emissions of an automobile engine by injecting water and an oxidant such as hydrogen peroxide into the engine""s combustion chambers. Engine exhaust gases are passed through a catalytic burner. Similarly, in U.S. Pat. No. 5,863,413 of Caren et al., hydrogen peroxide is partially dissociated into hydroxyl radicals and injected into an automobile engine carburetor, an engine exhaust manifold, or into the catalytic burner.
The addition of hydrogen peroxide to scrubbing liquors for pollutant removal is shown in U.S. Pat. No. 3,733,393 of Couillaud et al. and in U.S. Pat. No. 5,151,258 of Gubanc et al. The Couillaud et al. reference indicates that the incoming scrubbing liquors contain about 41% H2O2 for removal of SO2. The effectiveness of NOX removal is not indicated. The Gubanc et al. reference also indicates that a high concentration (0.5 to 10% or more) of hydrogen peroxide is added to the scrubbing liquor.
In a similar process described in U.S. Pat. 5,674,459 of Gohara, et al., flue gases are bubbled through diluted (18-20%) hydrogen peroxide containing recycled sulfuric acid and nitric acid. A portion of the liquors is drawn off and treated with limestone to produce gypsum.
In U.S. Pat. No. 5,670,122 of Zamansky et al., hydrogen peroxide or a mixture of hydrogen peroxide and methanol is injected into a flue gas stream. NO is converted to nitrogen dioxide NO2 which is subsequently reduced to N2 and removed.
Each of the processes indicated above has severe limitations. Those processes which achieve a relatively high removal of NOX have high capital and/or operating costs, making them generally unattractive. Processes with lower total costs do not achieve the desired high removal rates of NOX. In the United States, the payment of fines for excessive NOX emissions is the norm for operating plants in many industries.
In view of the foregoing background, it is therefore an object of the invention to provide a method and apparatus for achieving high removals of NOX from flue gases of stationary combustion sources and manufacturing plants at lower cost.
This and other objects, features and advantages in accordance with the present invention are provided by a method for removing NOX from gas streams emanating from stationary combustion sources and manufacturing plants wherein hydrogen peroxide is injected into the gas stream under conditions which will rapidly oxidize NOX species in gas-phase reactions. Nitric oxide NO is rapidly oxidized to nitrogen dioxide NO2. NO2 is further oxidized to nitrous acid HNO2 and nitric acid HNO3. These nitrogen oxyacids are much more water-soluble than nitric oxide NO (and even NO2), and may be removed by wet scrubbing of the oxidized gas stream, or by passing the oxidized gas stream through a particulate alkaline material to form a nitrite/nitrate salt. For example, electric power plants burning fossil fuels and using wet scrubbing to remove SO2 from the flue gas may be retrofitted so that enhanced simultaneous removal of NOX and SO2 is achieved.
Another aspect of the invention relates to an apparatus for removing NOX from a flue gas stream. The apparatus may include a reactor for injecting an oxidizing stream of hydrogen peroxide (H2O2) into the flue gas stream under gas-phase reaction conditions at which nitrogen oxides are oxidized to NO2 and at least one of water-soluble nitrogen oxyacids HNO2 and HNO3, and an acid remover downstream from the reactor for removing the water-soluble nitrogen oxyacids from the flue gas stream.